The 1997-98 El Niño event may have been a major contributor
in the average global sea level rising about 2 centimeters (0.8
of an inch) before it returned to normal levels, according to
scientists studying TOPEX/Poseidon satellite measurements of sea
surface height.

"This is the first time we have been able to identify that
El Niño may cause a change in average global sea level," said Dr.
R. Steven Nerem, a TOPEX/Poseidon science team member at the
Center for Space Research at the University of Texas at Austin.
"Understanding these short-term variations is important for
understanding and detecting long-term variations caused by
climate change."

"TOPEX/Poseidon measures average global sea level at 10-day
intervals with a precision of 0.4 centimeters (0.16 inches), so
detecting the 2-centimeter (0.8-inch) change associated with the
El Niño was relatively easy," Nerem said. "However, these
results tell us that detecting sea level variations caused by
climate change will be more difficult because such changes are
significantly smaller than the variations we have observed during
the El Niño."

Nerem and his colleagues are presenting their findings at
the American Geophysical Union's Fall Meeting in San Francisco
Monday, December 7.

Key to understanding the changes in the ocean are the global
maps made by TOPEX/Poseidon. The sea level rise was not confined
to the tropical Pacific, but also was observed in the Indian
Ocean and the southern Pacific. Nerem's team then calculated the
average global sea level.

"These six years of satellite data are a good start, but we
really need a decade or more of continuous measurements before we
can accurately detect any climate-induced change," said Dr. Lee-
Lueng Fu, the TOPEX/Poseidon project scientist at NASA's Jet
Propulsion Laboratory (JPL), Pasadena, CA. "We need sustained
observation records to understand the variations in the ocean."

Global mean sea level change on seasonal and inter-annual
time scales is a measure of the changing heat content of the
ocean. The 2-centimeter (0.8-inch) rise during the El Niño
implies that, on average, the global ocean may be gaining heat.

"Average global sea level began rising in late March 1997,
peaked at 2 centimeters (0.8 inches) above normal in early
November 1997, and then began falling back to normal by the end
of July 1998. Sea surface temperature changes began rising in
late October 1996, peaked at 0.4 degrees C (0.7 degrees F) in
late December 1997, and fell back to 0.1 degrees C (0.2 degrees
F) at present," according to Nerem.

Developed by NASA and the French Centre National d'Etudes
Spatiales (CNES), the TOPEX/Poseidon satellite, launched in
August 1992, uses an altimeter to bounce radar signals off the
ocean's surface to get precise measurements of the distance
between the satellite and the sea surface. These data are
combined with measurements from other instruments that pinpoint
the satellite's exact location in space. Every 10 days,
scientists produce a complete map of global ocean topography, the
barely perceptible hills and valleys found on the sea surface. A
follow-on mission to TOPEX/Poseidon, Jason-1, is scheduled for
launch in 2000.

JPL, a division of the California Institute of Technology,
manages the TOPEX/Poseidon mission for NASA's Earth Science
Enterprise, Washington, DC. The Earth Science Enterprise will
combine measurements like those from TOPEX/Poseidon with other
information about the land, sea, air, and life on Earth to
develop a greater understanding and predictive capability of the
global environmental system.